The present invention relates to a process for eliminating deposits formed in a steam generator of a pressurized water nuclear reactor. It more specifically relates to a process for eliminating deposits formed on the tube plate and in the gaps between the tubes and the spacer plates of a pressurized water nuclear reactor steam generator.
It is known that in a steam generator of this type, the primary fluid from the reactor circulates in a bundle of tubes fixed by expansion and welding in a tube plate which is positioned above the water box of the steam generator. These tubes are held in place by means of perforated spacer plates. During the operation of the reactor, steam generators lead to damage due to the deposition of oxides and corrosive products accumulating on the tube plate and in the gaps between the tubes and the spacer plates. The deposits on the tube plates permit the accumulation and concentration of noxious products, such as chlorides, sulphates and hydroxide ions. These products lead to the corrosion of the tubes, either through forming cracks, pitting or intergranular attacks. The oxides in the gaps between the spacer plates and the tubes can also be to the formation of a medium which is highly corrosive to the steel of the plates. The oxides produced by this corrosion lead to a contraction of the diameter of the tube, and this can lead to the cracking thereof.
This phenomenon, which is generally known under the name of "denting" and which will be examined in detail hereinafter makes it necessary to plug or seal the affected tubes.
These deposits cannot be mechanically eliminated because it is not possible to obtain access thereto due to the geometry of the steam generators. However, it is possible to act chemically thereon and several processes are presently used for this purpose.
Among the known processes making it possible to act chemically thereon and dissolve the corrosive products of the secondary circuit reference can be made to that involving the use of an ethylene diamine tetraacetic acid solution (EDTA).
The stages of this process are referred to hereinafter and it is possible to repeat the same and optionally combine a number of rinsing operations.
(a) Dissolving corrosion products deposited on the tube plate
The solution contains:
EDTA: 10% by weight PA1 Hydrazine: 1% by weight PA1 Corrosion inhibitor: 0.5% by weight PA1 EDTA: 5% by weight PA1 Hydrogen peroxide: 2% by weight PA1 EDTA: 20% by weight PA1 Corrosion inhibitor: 1% by weight
The pH is adjusted to 7.0 with ammonia. The contact time is 7 h at a temperature of 93.degree. C.
(b) Dissolving the copper contained in the deposited corrosion products
The solution contains:
The pH is adjusted to 7.0 with ammonia, and then to 10.0 using ethylene diamine. The contact time is 4 to 6 h at a temperature of 38.degree. C.
(c) Dissolving of corrosion products present in the gaps between the tubes and the spacer plates
The solution contains:
The pH is adjusted to 6.0 with ammonia. The contact time is approximately 120 h at a temperature of 121.degree. C.
Such a process suffers from the disadvantage of requiring a number of washing operations leading to large effluent quantities. Moreover, after a certain operating time, corrosion by pitting occurs, particularly in the case of manganese - nickel - molybdenum - steel forming the ferrules.
The object of the present invention is to eliminate these disadvantages by providing a washing process using a solution making it possible to dissolve the corrosion products present in the secondary circuit of a steam generator, without leading to the damage referred to hereinbefore.